Solid polymer ionic membranes or films have been well known in the art for many years. These polymers are typically characterized by high ionic conductivity, i.e., the rapid transport of ionic species, e.g., protons, at relatively modest temperatures, e.g., 50-90 degrees C. Additionally, it is desirable for such ionically conducting polymers to be made in the form of membranes or thin films. In so doing, the resistance to ionic transport, which is a function of the film thickness, can be reduced. Fluoropolymer compositions are particularly desirable for such uses, and are disclosed, for example, in U.S. Pat. Nos. 3,282,875, 4,358,545 and 4,940,525.
The instant invention relates to ionomers, which as used herein means a perfluorinated polymer containing acid groups or acid derivatives easily converted to acid groups such that the acid form of the polymer in membrane form has a room temperature ionic conductivity greater than 1×10−6 S/cm. As used herein the acid form of an ionomer means that substantially all the ion exchange groups, e.g., SO3− or sulfonic groups, are protonated. One important parameter used to characterize ionomers is the equivalent weight. Within this application, the equivalent weight (EW) is defined to be the weight of the polymer in acid form required to neutralize one equivalent of NaOH. As is known in the art, one can also convert the equivalent weight into other parameters that may be useful. For example the ion exchange capacity, which is 1000 divided by the equivalent weight; or the mole fraction or mole percent of ionomer in a copolymer of ionomer and non-ionomer. Higher EW means that there are fewer active ionic species (e.g., protons) present. If it takes more of the polymer to neutralize one equivalent of hydroxyl ions there must be fewer active ionic species within the polymer. Because the ionic conductivity is generally proportional to the number of active ionic species in the polymer, one would therefore like to lower the EW in order to increase conductivity.
Lowering the equivalent weight has previously not been a practical approach to making useful membranes. This is because with fluoropolymers currently known, as the equivalent weight goes down, the amount of water (or solvent) that the polymer absorbs goes up. The amount of water absorbed by the polymer is called the degree of hydration or hydration. It is expressed as the weight percent of water absorbed by the polymer under a given set of conditions, for example, after immersion in room temperature water for two hours. A higher degree of hydration is desirable up to a point because it tends to increase the ionic conductivity of the membrane. Correspondingly, lowering the degree of hydration has traditionally meant decreasing the conductivity. But there is a limit to the amount of water or solvent such fluoropolymer membranes can contain. If too much water is present, the film may lose much of its physical integrity, becoming gel-like with little or no rigidity. In the extreme, the polymer may completely disintegrate. In addition, depending on the exact polymer composition, low EW fluoropolymer ionomers may even partially or completely dissolve in water. Furthermore, even if the films were to be stable, too high a hydration would tend to dilute the number of ions present for conduction, thereby lowering the conductivity. Thus, there is an optimal degree of hydration that is high enough to provide the highest possible conductivity, while not so high that the films become physically unstable when hydrated.
Thus, one would like to decrease the equivalent weight of these fluoropolymers to increase their conductivity, but heretofore could not practically do so because the degree of hydration and/or water solubility was too high to form practical membranes.
Various approaches have been used to circumvent this limitation. In U.S. Pat. Nos. 5,654,109, 5,246,792, 5,981,097, 6,156,451, and 5,082,472 various forms of layered composite membranes are suggested. In '109, the use of a bilayer or trilayer composite ion exchange membranes is suggested where the outer layer or layers are lower equivalent weight for improved electrical performance, while the core layer has a higher EW that provides strength. A similar approach is suggested in '792 but the films are layers are characterized by their glass transition temperatures instead of EW. Three or more layers with variable ion exchange ratio (a parameter proportional to EW) is proposed in '097. In '472 a process to form a membrane is taught whereby a perfluorinated ionomer is laminated to a porous expanded PTFE membrane, followed by impregnation of a low equivalent weight ionomer (e.g., 920-950 EW) into that laminate. Because the impregnation is performed with a solution with low solids content (e.g., 2%), the amount of low equivalent weight material in the final product is relatively low. Although each of these approaches may offer some improvement over a monolithic single layer fluoropolymer membrane, they all involve the use of rather complex, composite, multilayer structures that can be difficult and/or expensive to process.
Approaches to modifying the fluoropolymer itself have also been taught, for example in U.S. Pat. No. 4,358,545 to Ezzell. The properties of these polymers are described in Moore and Martin, “Morphology and Chemical Properties of the Dow Perfluorosulfonate Ionomers”, Macromolecules, vol. 22, pp. 3594-3599 (1989), and Moore and Martin, “Chemical and Morphological Properties of Solution-Cast Perfluorosulfonate Ionomers”, Macromolecules, vol. 21, pp. 1334-1339 (1988). The approach described in these references is to produce ionomers with shorter side chains along the polymer backbone. This approach is particularly desirable for use in coating processes (for example, as described in U.S. Pat. Nos. 4,661,411 and 5,718,947), but still suffers limitations for use as fluoropolymer ionomer membranes. In particular, these polymers can still be difficult to form into acceptably thin, strong membranes from solution.
Another approach as described by various authors is to form co-polymers of tetrafluoroethylene and ionomers using variations of the well-known emulsion polymerization (for example, the process disclosed in U.S. Pat. No. 3,282,875). In U.S. Pat. No. 5,608,022 to Nakayama et. al. and WO 00/52060 to Bekarian, et. al., processes are taught to form functionalized, fluorinated co-polymers by dispersing fine droplets of a fluorinated co-monomer before polymerizing with a traditional fluorine containing monomer, e.g. tetrafluoroethylene. In these processes, the formation of fine droplets of the co-monomer is a key to a successful preparation of the polymer. In WO 94/03503 to Barnes, et. al. the rate of addition of the tetrafluoroethylene monomer to the ionomer emulsion is controlled by either altering the concentration of the emulsion during polymerization, varying the pressure of the tetrafluoroethylene gas during reaction, or varying the agitation of the reaction mixture. Barnes teaches that these approaches result in a product with higher utilization of the ionomer as determined by the property of equivalent weight distribution, which he defines as a ratio of EW determined by means of titration to that determined by nuclear magnetic resonance. Barnes et. al. claims that this higher utilization leads to a higher Relative Hydration Product and higher Specific Conductivity. Both these parameters were evaluated in the presence of 2.5 Molar sulfuric acid (2.5 M H2SO4), and therefore are not relevant to the current application where only hydrated polymer (in the absence of acid electrolyte) is considered.
In yet another approach taught in PCT WO 00/79629 an ionomeric polymer is intimately mixed with a structural film-forming polymer, such as a terpolymer of tetrafluoroethylene, hexafluoropropylene and vinylidene fluoride (e.g., THV series available from Dyneon Corp., Oakdale, Minn.). It then is possible to form acceptably thin films using low equivalent weight ionomers. But, the degree of hydration is still relatively high, 80-110%, when 800 EW starting ionomer is used (e.g., Table 1 in WO 00/79629). Thus, these films might be expected to be relatively weak because of the high hydration.
Finally there is also a large body of art that describes approaches to forming non-ionomeric fluoropolymers. For the most part, this art is not relevant to the instant invention described here because the products produced do not have substantial ionic conductivity, i.e., the ionic conductivity of these products is less than about 1×10−6 S/cm at room temperature.
Fluoropolymer ionically conducting membranes have been utilized in many different applications. One application that has been widely suggested is as electrolytic cell membranes for the electrolysis of sodium chloride as disclosed, for example, in U.S. Pat. Nos. 4,358,545, 4,417,969, and 4,478,695. Additionally, this generic class of polymers described as fluoropolymer ionomers have been proposed for use as coatings as described above in U.S. Pat. No. 4,661,411 to Martin et al.; as wire insulation (e.g., in WO 90/15828); as replacements for acid catalysts, primarily in organic synthesis as described in “Perfluorinated Resin sulfonic Acid (Nafion-H (R)) Catalysis in Synthesis”, by Olah, G. A., Iyer P. S. and Surya P. G. K., in Journal: Synthesis (Stuttgart), 1986 (7) 513-531, and in “Perfluorinated Resin sulfonic acid (Nafion-H) Catalysis in Organic Synthesis” by Yamato, T., in Yuki Gosei Kagaku Kyokaishi/Journal of Synthetic Organic Chemistry, volume 53, number 6, June 1995, p 487-499; as a membrane for water electrolysis as described in Yen, R. S., McBreen, J., Kissel, G., Kulesa, F. and Srinivasan, S. in the Journal of Applied Electrochemistry, volume 10, pg. 741, 1980; as a membrane for electrowinning as described, for example, in “The Use of Nafion-415 Membrane in Copper Electrowinning from Chloride Solution” by Raudsepp, R., and Vreugde, M., in CIM Bulletin, 1982, V75, N842, P122; in metal ion recovery systems as described in product literature of Nafion® perfluorinated membrane case histories, DuPont Company, Polymer Products Department, Wilmington, Del. 19898; as a tube to continuously and very selectively dry wet gas streams (see product literature from Perma Pure, Inc., Toms River, N.J.); and as components in polymer electrolyte membrane (PEM) fuel cells. In the latter case, they can function both as the electrolyte or a component thereof, for example as described in by Bahar et.al. in U.S. Pat. Nos. 5,547,551 and 5,599,614; and/or as a component in one or both of the electrodes of the MEA.
When the ion conducting polymers, or ionomers, are used as the electrolyte in PEM fuel cells they conduct protons from one electrode to the other. A common problem associated with such fuel cells is that contaminants such as carbon monoxide tend to poison the catalysts used in the MEA. These contaminants can interfere with the flow of ions between the electrodes and thus degrade the performance of the fuel cell.
One way to reduce the effect of carbon monoxide is to operate the fuel cell at an elevated temperature. This reduces the formation and/or increases the destruction rate of potential contaminants and thereby allows more efficient electrode performance.
The problem with running at high temperatures, however, is that it vaporizes liquid water within the fuel cell, and in so doing, tends to reduce the degree of hydration in the membrane. As described above, decreasing the hydration lowers the ionic conductivity, thereby reducing the efficiency of ion transport through the membrane and adversely affecting fuel cell operation. In fact, at lower temperatures, in PEM fuel cells using conventional ionomers the incoming gas streams are usually well-humidified in order to maintain a relatively high degree of hydration. Only by adding the additional water in the form of humidity in the gases can the hydration be kept high enough to allow efficient fuel cell operation for long periods of time. However, as the temperature gets close to, or above, the boiling point of water this approach becomes difficult and inefficient. Thus, an ionomer with relatively low hydration and acceptably high ionic conductivity would require less ambient water to function as the electrolyte in PEM fuel cells. It could function efficiently both in lower humidity environments at lower temperatures, as well as at temperatures closer to and even potentially above the boiling point of water.
As described above, the known low equivalent weight ionomers have a relatively high hydration. They are also known to be partially or completely soluble in water as well. These factors would counsel against their use in environments where water is produced, e.g. hydrogen-oxygen fuel cells, because these polymers tend to become physically unstable in these environments. In addition, as described above and shown recently (WO 00/52060, Table 1) the ionic conductivity decreases as the equivalent weight goes down concomitant with a large increase in hydration. The ionic conductivity reported in WO '060 decreases by more than 30% when the equivalent weight of the subject ionomer is reduced from 834 to 785.
Against this background of conventional wisdom, applicants have discovered a low equivalent weight ionomer that has a combination of very high ionic conductivity while maintaining a relatively low hydration. As a result, this invention makes possible the more effective use of solid fluoropolymer membranes in existing applications such as those described above. Additionally, new applications heretofore not practical may become possible with this new, unique set of characteristics. The instant invention is particularly valuable as an electrolyte or component thereof, or as a component in the electrode of polymer electrolyte membrane fuel cells operating at high temperatures or low humidities.